In the past, a scrambler has been provided to encode premium television channels at a headend of a cable television system. The applied scrambling precluded reception by an unauthorized converter/decoder at a connected premises. Data representing the channels or tiers of programming to which the subscriber was entitled were addressably transmitted to a particular converter/decoder and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program would be authorized by selectively enabling the decoder portion of the converter/decoder to decode the scrambled premium channel or program.
The provision of one scrambler per premium channel at the headend and the inclusion of a descrambler in each converter/decoder at the premises of the television receiver was particularly expensive. Furthermore, providing a converter/decoder on premises has turned out to be a great temptation to service pirates who imaginatively seek ways to receive premium channels. As a result, cable television equipment manufacturers have entered into a veritable war with such pirates resulting in complicated service authorization protocols, which in some instances involve multiple layers of encryption by both in-band and out-of-band data transmissions thereby further increasing the costs of the converter/decoder. In addition, scrambling systems may leave artifacts in the final signal.
Consequently, the cable industry has reviewed other technology developed in the early stages of cable television, such as the application of negative and positive traps, and more recent techniques, such as interdiction, to improve CATV systems.
Negative trap technology is viewed by many manufacturers as a viable alternative to scrambling methods. A negative trap is basically a narrow band rejection filter. Traps are located at the drop to a subscriber's dwelling and attenuate a significant portion of a premium television channel rendering that channel unusable by the subscriber. Recently, a new type of negative trap has been introduced. The dynamic negative trap consists of a notch filter that is designed to be modulated with respect to frequency. The notch is centered about the picture carrier but is deviated slightly from side to side. The television channel is rendered unusable by the attenuation and by the introduction of unwanted amplitude and phase modulation of the picture carrier.
Positive trap systems also utilize a narrow band rejection notch filter. However, unlike negative trap systems which are used to attenuate or trap a premium channel transmission, the notch filter is used to restore the premium television channel. In this scenario, an interfering signal is placed on the premium television channel at the cable television headend This interfering signal is then removed at the subscriber by the use of the notch filter. Ideally, this notch filter removes only the interference without removing a significant amount of the television information.
Parallel to developments of different types of trapping and jamming systems, the cable industry has also evidenced a requirement to move a converter/decoder outside of a subscriber's home to a location which is more secure from signal piracy. For example, an addressable tap system was developed by Scientific Atlanta in which an off-premises "tap", addressed by a headend control system, controlled premium channel series into the subscriber's premises. However, such addressable taps did not prove to be viable alternative to the inside-the-home signal converter/decoder.
A relatively recent technique for premium channel control is the interdiction system, so called because of the introduction of an interfering signal into a premium channel at the subscriber's location. Most embodiments consist of a pole-mounted enclosure located outside the subscriber's premises designed to serve one or more subscribers. This enclosure contains at least one microprocessor controlled oscillator and switch control electronics to secure several television channels. Control is accomplished by injecting an interfering or jamming signal into unauthorized channels from this pole-mounted enclosure.
For the sake of efficiency, it is known to utilize one oscillator to jam several premium television channels. This technique not only reduces the amount of hardware required, but also maximizes the system flexibility. The jamming signal frequency is moved as a function of time from channel to channel. The oscillator is frequency agile and hops from jamming one premium channel frequency to the next. Cable television channels and, of course, premium service channels may extend over a wide range of frequencies, for example, from 54 MHz. to 550 MHz. Thus, if only one oscillator is provided, it must be frequency agile over a wide range.
One such system is known from U.S. Pat. No. 4,450,481 in which a single frequency agile oscillator provides a hopping gain-controlled jamming signal output to four high frequency electronic switches. In this system, each switch is associated with one subscriber drop. Under microprocessor control and depending on which subscribers are authorized to receive transmitted premium programming, the microprocessor selectively gates the jamming signal output of the single oscillator via the switches into the path of the incoming broadband television signal to each subscriber. Consequently, an unauthorized subscriber upon tuning to a premium channel will receive the premium channel on which a jamming signal at approximately the same frequency has been superimposed
It is indicated that the jamming signal is at a high relative power and is gain controlled to exceed the amplitude of the video carrier by 5 to 20 dB. Because of the high output power relative to the premium channel video carrier power and the difficulty of precisely jamming the premium channel frequency, such an interdiction system leaves considerable opportunity for improvement. Because the oscillator is frequency hopping, its spectrum tends to spread out around the picture carrier, generating a slightly different situation as far as the required adjacent channel rejection characteristics of the television signal is concerned.
Additionally, it is important in an interdiction system that the jamming signal be properly matched in level with the picture carrier level of an interdicted channel. Furthermore, this match is important not only to compensate for drift in the components due to temperature variations and seasonal weather changes but to also compensate for level variations due to its location in a CATV distribution plant and to compensate for tilt due to imperfect gain requirements of a distribution cable over the frequency spectrum. Otherwise, adjacent channel artifacts or incomplete jamming will result. In the previous system, conventional gain sensing and control circuits are used for gain control to compensate only for the simplest of variations.
Not only in conventional interdiction systems but also in accordance with grandparent application U.S. Ser. No. 279,619 the jamming carrier level was matched to the incoming picture carrier level. In that application, it was suggested to improve gain control by sampling the picture carriers at the high and low ends of the frequency spectrum at pilot frequencies so as to be better prepared to regulate the jamming carrier amplitude level to match the level of the incoming picture carrier. Furthermore, it was suggested that a slope characteristic for the particular cable distribution plant could be downloaded to an interdiction unit in the vicinity of the subscriber for improved frequency compensation control.
In a normal CATV subscriber installation, regardless of whether negative trap, positive trap or interdiction is applied, the picture carrier signal level can be expected to vary by several decibels over time and temperature. This variance may result from losses in the distribution cable, errors in the automatic gain control circuits of distribution amplifiers, aging of electronic components, and other effects.
From CATV distribution amplifiers operating on trunk lines, it is known to sense the level of a picture carrier after a variable gain element and adjust the gain of the variable gain element until the picture carrier level sensed is equal to a predetermined reference level established by the configuration of the cable distribution plant. Consequently, while controlling the gain of a picture carrier is known from the design of cable distribution systems, no previous consideration was given to controlling the gain of a picture carrier in a jamming or interdiction system at a subscriber.
Typically, in the conventional systems previously alluded to, the gain control for matching the jamming carrier level to the incoming picture carrier level is carried out on a per-subscriber basis. This is a costly procedure, and there remains a requirement to reduce costs wherever possible without jeopardizing the quality of any television signal received at a customer's premises.
In parent application U.S. Ser. No. 446,603 it is disclosed that a more advantageous method of compensating for cable distribution plant characteristics and home installation variations for an off-premises CATV system is to control the picture carrier level with respect to the jamming carrier level. This provides a predetermined relationship between the picture carrier level and the jamming carrier level without the necessity of addressing individual subscriber installations. Moreover, in the parent application there is taught that automatic gain control of the picture carrier level can be accomplished by measuring the broadband television signal power level with a bandpass filter. This reduces the cost of the circuitry which is an important consideration for subscriber equipment.
With respect to the tilt correction in this system, a fixed network was utilized. This required that installation personnel measure the characteristics of the cable distribution plant at the particular location and select the correct values for the network. While providing substantial tilt correction, this method produces uneven results because of the human errors in making a relatively difficult measurement and selecting the correct components to install
It would be advantageous to provide an automatic slope control circuit for an interdiction apparatus. Prior automatic compensation circuits for trunk amplifiers have used two pilot frequencies to determine the amount of compensation for a broadband television signal. A pilot frequency on one end of the CATV band is measured to control gain and a second pilot frequency on the other end of the band is measured to control slope. This circuit requires relatively expensive bandpass filters for the measurement of the pilot frequencies and further requires that the filters be set to detect frequencies which are always present. The cable operator must always present a carrier on the chosen channel. While this is not too burdensome when talking about trunk amplifiers where the per unit cost can be relatively large because there are so few of them, it becomes much more of a problem when providing an automatic slope control for interdiction apparatus because of the number of units. This problem will increase with the growing popularity of interdiction units for single subscribers where one automatic control per premises is needed.
Consequently, prior to the present invention, the need remained for improved compensation in off-premises control systems which alleviate the effects of cable distribution plant characteristics in view of variations in home installations. Furthermore, any such automatic compensation control should be provided in a cost-effective manner without jeopardizing signal quality.